North Cascades Geology

As the crust is deformed by northward drift a granitic dike is: A. faulted near the surface (and viewed after some erosion), then...

Caulking the Cracks

Rocks in the deeper crust remained hot and plastic during extension and northward drift. They stretched and recrystallized. At shallow levels the cool, brittle crust broke as it extended and western pieces drifted northward.

B. stretched like taffy at depth where the rocks are hot (and viewed only after much erosion).

During the later phases of drifting and extension, about 45-50 million years ago (early Tertiary), plumes of granite magma rose into the deforming crust. One of these plumes of ascending magma became the Golden Horn Batholith. It caulks part of the Ross Lake fault, one of the faults along which crustal blocks migrated north. During this magmatic phase the Skagit Gneiss Complex, still buried deeply enough to remain hot and pliable, was intruded by innumerable granite sills and dikes which were stretched into lineated orthogneiss.

Summing up: Why extension?

The crust can extend because its strength decreases and it collapses of its own weight (think of a softening block of butter), or because it is pulled apart. Or both. Geologists think that the 55 to 40 million-year-old (Eocene) extensional deposits of the North Cascades are mostly the result of the crust being pulled apart by the plate tectonic forces that for tens of millions of years previously had been moving the terranes northward. Softening may also have contributed to the extension because a considerable mass of the accreted terranes was very hot, undergoing metamorphism, and lubricated by igneous plutons (which became the orthogneisses). A slight change in the pressures of the adjoining plates, and the whole structure would have collapsed.

The extensional event may have been initiated by re-arrangement of plate motions in the adjacent Pacific Ocean; there is evidence that the rate of plate motion increased at this time, and perhaps a different plate came into contact with North America at the latitude of the North Cascades. The extensional event, most (but not all) of the northwards drift of western crustal blocks, and distinctive granitic magmatism came to an end when plates in the Pacific Ocean again rearranged about 40 million years ago and the Cascade Volcanic Arc was born.

Material in this site has been adapted from a book,
Geology of the North Cascades: A Mountain Mosaic by R. Tabor
and R. Haugerud, of the USGS, with drawings by Anne Crowder. It is published
by The Mountaineers, Seattle.